Regarding the warning devices for the dripping bottle in intravenous injection in the prior art, there are Spani's "Empty Container Detectar" U.S. Pat. No. 4,665,391; jinotti's "Fluid-dispensing Apparatus Having Level Control and Alarm Means" U.S. Pat. No. 4,014,010; Deltour's "Drop Monitor", etc. Among the aforesaid inventious, Spani's and Jinotti's inventious are used to detect the fluid level scales by means of photoelectric transducers being installed across a fluid container; one of the photoelectric transducers is associated with the fluid container above the fluid level in the container to establish a reference signal corresponding to the condition wherein the light path is not refracted or attenuated by the presence of a fluid. Electrical means are provided to connect the first photoelectric transducer to the second photoelectric transducer to linearize and normalize the reference signal with a control signal generated by another photoelectric transducer. A comparator is associated with the photoelectric transducer, and connected to an alarm, which indicates when the control signal is substantially equivalent to the reference signal, and thus indicating the lowering of the fluid level below the light path of the first photoelectric transducer.
In Deltour's (U.S. Pat. No. 3,563,090), the structure and design of the "Drop Monitor" is similar to Spani's invention. The light source and the photo-resistive cell are installed at one side of the drop observation tube 24 instead of being installed on the same level. On the opposite side of the light source and the photo-resistive cell, there installed a mirror so as to let a light pass through a medium surface, and then the light is reflected back to the photo-resistive cell.